The objective of this project is the development of a 64Cu (T1/2 = 12.7 h) PET (Positron Emission Tomography) radiopharmaceutical for the functional evaluation of multidrug resistance (MDR), a frequently encountered treatment-limiting phenomenon in which a malignancy fails to respond to or becomes resistant to a specific class of drugs. MDR is characterized by increased concentrations of P-glycoprotein (Pgp) and multidrug-resistance protein (MRP1), which reduce the concentration of these drugs in the resistant cells. Our objective is based on the hypothesis that a lipophilic cationic 64Cu PET radiopharmaceutical can be developed with biological properties that are equal or superior to those of existing 99m/Tc SPECT (Single-Photon Emission Computed Tomography) radiopharmaceuticals that are known to be substrates for Pgp and MRP1, and, furthermore, that the use of PET for this procedure will improve the diagnostic accuracy compared to SPECT. The Specific Aims of this project are: 1) the identification of the optimal chemical properties of this radiopharmaceutical, and 2) the characterization of its biological properties. The first Aim will be accomplished by synthesizing a series of targeted derivatives of the lipophilic cationic diiminedioxime 64Cu complex that we have previously demonstrated is a substrate for Pgp. These derivatives will be chosen based on the properties of existing MDR radiopharmaceuticals as well as the structure-function relationships that we develop in the course of this project. The second Aim will be accomplished by measuring the uptake of the 64Cu complexes in resistant and non-resistant cell lines, the effect of MDR modulators, the serum stability, and the non-specific binding in vitro. The uptake of the complexes by resistant and non-resistant tumors will also be measured in vivo as will the in vivo stability. These structure-function relationships will then be applied to the synthesis of subsequent 64Cu complexes. The successful development of a PET radiopharmaceutical for the quantitative functional assay of MDR will provide a valuable clinical tool for real-time assessment of MDR, which may facilitate optimization of chemotherapy protocols and evaluation of MDR reversal agents.